Subject: Re: [RFC PATCH net-next 1/3] net: phy: don't bind genphy in phy_attach_direct if the specific driver defers probe

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 2 Sep 2021 23:24:39 +0100

On Fri, Sep 03, 2021 at 12:39:49AM +0300, Vladimir Oltean wrote:
> On Thu, Sep 02, 2021 at 10:33:03PM +0100, Russell King (Oracle) wrote:
> > That's probably an unreliable indicator. DPAA2 has weirdness in the
> > way it can dynamically create and destroy network interfaces, which
> > does lead to problems with the rtnl lock. I've been carrying a patch
> > from NXP for this for almost two years now, which NXP still haven't
> > submitted:
> > 
> > http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=cex7&id=a600f2ee50223e9bcdcf86b65b4c427c0fd425a4
> > 
> > ... and I've no idea why that patch never made mainline. I need it
> > to avoid the stated deadlock on SolidRun Honeycomb platforms when
> > creating additional network interfaces for the SFP cages in userspace.
> 
> Ah, nice, I've copied that broken logic for the dpaa2-switch too:
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d52ef12f7d6c016f3b249db95af33f725e3dd065
> 
> So why don't you send the patch? I can send it too if you want to, one
> for the switch and one for the DPNI driver.

Sorry, I mis-stated. NXP did submit that exact patch, but it's actually
incorrect for the reason I stated when it was sent:

https://patchwork.ozlabs.org/project/netdev/patch/[email-masked]/

I did miss the rtnl_lock() around phylink_disconnect_phy() in the
description of the race, which goes someway towards hiding it, but
there is still a race between phylink_destroy() and another thread
calling dpaa2_eth_get_link_ksettings(), and priv->mac being freed:

static int
dpaa2_eth_get_link_ksettings(struct net_device *net_dev,
                             struct ethtool_link_ksettings *link_settings)
{
        struct dpaa2_eth_priv *priv = netdev_priv(net_dev);

        if (dpaa2_eth_is_type_phy(priv))
                return phylink_ethtool_ksettings_get(priv->mac->phylink,
                                                     link_settings);

which dereferences priv->mac and priv->mac->phylink, vs:

static irqreturn_t dpni_irq0_handler_thread(int irq_num, void *arg)
{
...
        if (status & DPNI_IRQ_EVENT_ENDPOINT_CHANGED) {
                dpaa2_eth_set_mac_addr(netdev_priv(net_dev));
                dpaa2_eth_update_tx_fqids(priv);

                if (dpaa2_eth_has_mac(priv))
                        dpaa2_eth_disconnect_mac(priv);
                else
                        dpaa2_eth_connect_mac(priv);
        }

static void dpaa2_eth_disconnect_mac(struct dpaa2_eth_priv *priv)
{
        if (dpaa2_eth_is_type_phy(priv))
                dpaa2_mac_disconnect(priv->mac);

        if (!dpaa2_eth_has_mac(priv))
                return;

        dpaa2_mac_close(priv->mac);
        kfree(priv->mac);		<== potential use after free bug by
        priv->mac = NULL;		<== dpaa2_eth_get_link_ksettings()
}

void dpaa2_mac_disconnect(struct dpaa2_mac *mac)
{
        if (!mac->phylink)
                return;

        phylink_disconnect_phy(mac->phylink);
        phylink_destroy(mac->phylink);	<== another use-after-free bug via
					    dpaa2_eth_get_link_ksettings()
        dpaa2_pcs_destroy(mac);
}

Note that phylink_destroy() is documented as:

 * Note: the rtnl lock must not be held when calling this function.

because it calls sfp_bus_del_upstream(), which will take the rtnl lock
itself. An alternative solution would be to remove the rtnl locking
from sfp_bus_del_upstream(), but then force _everyone_ to take the
rtnl lock before calling phylink_destroy() - meaning a larger block of
code ends up executing under the lock than is really necessary.

However, as I stated in my review of the patch "As I've already stated,
the phylink is not designed to be created and destroyed on a published
network device." That still remains true today, and it seems that the
issue has never been fixed in DPAA2 despite having been pointed out.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!